IONIZATION NEBULAE SURROUNDING CAL-83 AND OTHER SUPERSOFT X-RAY SOURCES

We present the results of an optical search for ionized gaseous nebula e surrounding luminous, ''supersoft'' X-ray sources in the Large and S mall Magellanic Clouds. This relatively new and mysterious X-ray class has characteristic luminosities similar to 10(37)-10(38) ergs s(-1) w ith effective temperatures in the range of 2-6 x 10(5) K. The presence of a large flux of UV and soft X-ray photons from these objects has l ed to predictions of bright optical emission lines from the local inte rstellar medium. One such object, CAL 83 in the LMC, was known to have an associated nebula, and we quantify here the asymmetry and luminosi ty of this remarkable nebula. Deep images were made using narrowband f ilters to isolate the emission lines of H alpha and [O III] (gamma 500 7). In these emission lines, the nebula is detected out to distances a s far as 25 pc from the central object, and the integrated luminosity in each line is of order similar to 100 L.. Model calculations of such nebulae for chemical abundances characteristic of the LMC indicate th at similar to 1% of the X-ray luminosity of the central source is repr ocessed into the nebular H alpha and [O III] gamma 5007 emission lines , from which we conclude that the time-averaged X-ray luminosity of th e central source, CAL 83, is greater than 3 x 10(37) ergs s(-1). The b right inner nebula contains similar to 150 M. within 7.5 pc of CAL 83, which clearly indicates that the nebular material has its origin in t he interstellar medium. In sharp contrast, there were null detections for nebulae associated with nine other luminous, supersoft X-ray sourc es in the LMC and SMC, with upper limits for the [O III] luminosity th at are a factor of similar to 10 below that for CAL 83. For eight of t hese latter sources, we conclude that either their time-averaged lumin osity is substantially below that of CAL 83, or that the local interst ellar medium is much less dense. The latter effect may be enhanced by expansion of the ionized nebula during the last several million years.